No　need to い　adjective change to Noun as like "美しい”　＋　もの　+　だ　　　楽しい　＋　こと　+だ　 is the way use with " だ”　　い　adjective 's conjugation form connect with other than Noun form like うつくしか　＋　った　　　たのしか＋った　　　change to Ta form ~ った　　or うつくしく＋なる　　楽しく＋　　なる　Those making formed those two type and なる’s　たform is なった Therefore if you intend to　use "だ It need to change into Noun (+ もの、こと　）　after that you have to added "だ”